1. Field of the Invention
The present invention relates to a communication apparatus and method which perform Media Access Control on the basis of the carrier sense information of a physical layer and the carrier sense information of a MAC layer.
2. Description of the Related Art
The Media Access Control (MAC) is control for causing a plurality of communication apparatuses which perform communication while sharing the same wireless media to decide how to use the wireless media in transmitting communication data or management frame. Owing to Media Access Control, even if two or more communication apparatuses transmit communication data or management frame by using the same wireless media at the same time, there is less chance of the occurrence of a phenomenon (collision) in which a communication apparatus on the receiving side cannot decode communication data properly. Media Access Control is also a technique for controlling access from communication apparatuses to a media so as to minimize the chance of the occurrence of a phenomenon in which, despite the presence of communication apparatuses having transmission requests, the media is not used by any of the communication apparatuses.
However, especially in wireless communication, it is difficult to simultaneously monitor transmission data (or management) frame while the communication apparatus transmits the frame, and therefore the Media Access Control (MAC) is required in which collision detection is not assumed. A typical technique standard of wireless LAN IEEE802.11 adopts CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The CSMA/CA is designed to reduce the collision probability. In IEEE802.11, the MAC header has the Duration value which is the time, in microseconds, required to transmit the data or management frame (including the time of SIFS interval). In this period, a communication apparatus which is irrelevant to the frame exchange sequence and has no transmission right waits for transmission upon determining a virtual busy state of the media. This prevents the occurrence of collision. IEEE 802.11 defines that the state of a wireless media is determined on the basis of such a combination of virtual carrier-sense on a MAC layer and physical carrier sense on a physical (PHY) layer, and Media Access Control is performed on the basis of the determination.
IEEE 802.11 using CSMA/CA has increased the communication speed mainly by changing the physical layer protocol. With regard to the 2.4 GHz band, there have been changes from IEEE 802.11 (established in 1997, 2 Mbps) to IEEE 802.11b (established in 1999, 11 Mbps), and further to IEEE 802.11g (established in 2003, 54 Mbps). With regard to the 5 GHZ band, only IEEE 802.11a (established in 1999, 54 Mbps) exists as a standard. In order to develop standard specifications directed to further increase communication speeds in both the 2.4 GHz band and the 5 GHz band, IEEE 802.11 TGn (Task Group n) has already been established.
In addition, several access control techniques designed to improve QoS (Quality of Service) are known. For example, as a QoS technique of guaranteeing parameters such as a designated bandwidth and delay time, HCCA (HCF Controlled Channel Access) which is an extended scheme of a conventional polling sequence is available. According to HCCA, scheduling is performed in a polling sequence in consideration of required quality so as to guarantee parameters such as a bandwidth and delay time. Jpn. Pat. Appln. KOKAI Publication No. 2002-314546 refers to QoS in the IEEE 802.11e standard, and discloses a method of assigning priorities to communications between communication apparatuses in a wireless network.
When the same frequency band as that in the existing specifications is to be used in realizing an increase in communication speed, it is preferable to assure coexistence with communication apparatuses conforming to the existing specifications and to maintain backward compatibility. For this reason, it is basically preferable that a protocol on a MAC layer conforms to CSMA/CA matching the existing specifications. In this case, a temporal parameter associated with CSMA/CA, e.g., an IFS (InterFrame Space) or random backoff period needs to match that in the existing specifications.
Even if an attempt to increase the communication speed in terms of physical layer succeeds, the effective throughput of communication cannot be improved. That is, when an increase in the communication speed of the physical layer is realized, the format of a PHY frame (PHY preamble and PHY header) ceases to be effective any more. An increase in overhead due to this may hinder an increase in throughput. In a PHY frame, a temporal parameter associated with CSMA/CA is permanently attached to a MAC frame. In addition, a PHY frame header is required for each MAC frame.
As a method of reducing overhead and increasing throughput, a Block Ack technique introduced in recently drafted IEEE 802.11e/draft 5.0 (enhancement of QoS in IEEE 802.11) is available. The Block Ack technique can consecutively transmit a plurality of MAC frames without any random backoff, and hence can reduce the backoff amount to some degree. However, a physical layer header cannot be effectively reduced. In addition, according to aggregation introduced in initially drafted IEEE 802.11e, both the backoff amount and the physical layer header can be reduced. However, since the length of a physical layer frame containing MAC frames cannot be increased beyond about 4 kbytes under the conventional limitation on the physical layer, an improvement in efficiency is greatly limited. Even if the length of a PHY layer frame can be increased, another problem arises, i.e., a reduction in error tolerance.